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Report:Groundwater-quality Monitoring-November 28,2000: 7500 West Eleventh Street, Tracy, CA. Page 7 diameter well boring is 14.56 gallons. It is discrepancies such as these, together with other factors such as those discussed above, that result in the widely disparate results j { reported in the literature when attempts are made to correlate concentrations of analytes '? of concern in a groundwater sample with the number of"well volumes" that have been purged from the well. :j V It has been SJC's experience, after characterizing a large number of sites in a wide range of hydrostratigraphic settings where groundwater has been affected by a variety of contaminants that, in purging shallow wells in groundwater affected by hydrocarbons, it is good practice to remove a true well volume of water (i.e., the total volume of water in the submerged well boring over the depth where the well construction includes a screen f:. and annular filter pack) before a groundwater sample is recovered. This protocol is of particular importance where groundwater levels vary seasonally and where soils around the water table affected by fuel hydrocarbons are successively submerged and then .: stranded in the vadose zone where they can be a source of seepage into filter pack material. Overly-aggressive well purging can present problems. In cases where down-gradient migration of plumes of fuel hydrocarbons result in large changes in the concentrations of the analytes of concern over short distances (as is the case at the 7500 West Eleventh Street :site), excessive purging can draw contaminated water having different concentrations of analytes of concern into a well so that the groundwater-quality assessed from a sample recovered from the purged well is not representative of that which actually F _l; prevails at that well location. Because the quantity of water that needs to be purged from the wells at 7500 West Eleventh Street is so small (less than 2.5 gallons represents more than one submerged casing volume), a system based on monitoring of indicator parameters of a purge pump discharge can easily led to such over-purging. This is primarily due to the fact that the volume of water trapped in the pump body, the 3 discharge piping, and other components of the purging equipment, together with the volume of flow that must occur for indicator parameters to stabilize after the pump is started is very large relative to the optimum purge volume. By the time indicator �. parameters in the purge water stream have stabilized after the sudden perturbation in the hydraulic regime that occurs when the pump is started, the well may have been fully purged of stagnant water. Thus, for the reasons discussed above, to achieve representative groundwater sampling without unnecessary technical complication and unjustifiable, increased sampling costs, SJC strongly recommends that, for future rounds of groundwater sampling at the 7500 'i_-. West Eleventh Street site, the method of well purging be restored to SJC's standard procedure for this site: each well is to be purged of 15 gallons of water or until the well F runs dry, whichever comes first. This procedure greatly exceeds the minimum k: requirement to purge the wells of "three well volumes" (i.e., three submerged casing volumes) that is cited as an alternative procedure to monitoring purge water stream parameters in SJCPHS's guidance letter of September 19, 2000 or the "three to five well volumes" cited in USEPA guidance documents. (United States Environmental Protection ( + Agency 1991) sic